This is a continuing study of the structure, connections, and relations of cell types in the primary auditory cortex (AI) of the cat. Its chief goal is to identify the neural circuits for three kinds of sensory information processing, and then to compare these findings with the pattern of organization already established in the somatic sensory and visual cortex. Input from thalamic neurons probably reaches certain cell types in layers III and IV and AI; the neurons postsynaptic to these axons will be studied in both the light and electron microscope using various axoplasmic transport and degeneration techniques. The morphology of the neural populations in these layers has already been described in normal material, and the present work is designed to provide complementary evidence about connections and ultrastructure. The second set of experiments will address the commissural connections, also using transport and degeneration methods. The objective is to identify the cells of origin for this system, and to determine if these cells receive reciprocal commissural input as well. The major patterns of synaptic endings will also be characterized with regard to their laminar patterns, shape and size, vesicle morphology, postsynaptic targets, and characteristic form of degeneration after lesions. A major goal of these two facets of the project is to relate the form and connections of individual cells to the columnar organization of axons and dendrites already described in AI. The third project examines the intrinsic connections of AI with itself and its association connections with adjoining ipsilateral auditory cortical areas. Various combinations of tracers will be used to examine patterns of overlap or interdigitation to compare how, for example, association pathways are related to commissural input patterns, since it is apparent that AI process input from each of these disparate sources. Besides achieving some intellectual parity in AI to make it directly comparable to somatic sensory and visual cortex, these findings have significance for an understanding of the structural basis for discriminative, binaural, and feature extraction mechanisms in hearing. Dysfunction or modification of these pathways might be implicated in acoustic trauma, aging, or environmentally related hearing loss; the present work could suggest morphological bases for pathological changes, and potential avenues for future clinical intervention.